In recent years, the downsizing of a magnetic circuit component such as a reactor or a transformer used in a power supply circuit is urged strongly with an increasing demand for the downsizing of a switching power supply. In a switching power supply, as a method of materializing downsizing, higher frequency is attempted in some cases. Eddy-current loss caused by skin effect, proximity effect, or leakage flux in a copper wire or a coil increases with the increase of frequency and thus a problem here is that the alternating-current resistance of a coil increases. When an alternating-current resistance increases, loss generated in a coil increases and hence a significant deterioration in the efficiency of a switching power supply is concerned.
On this occasion, with regard to the loss, when a higher frequency is taken into consideration as a premise, an alternating-current resistance increases in proportion to the one-half power of a frequency in the case of the skin effect. In comparison with the skin effect in contrast, an eddy-current loss increases in proportion to the square of a frequency in the case of an alternating-current resistance caused by a leakage flux. Consequently, a challenge on the occasion of higher frequency is to restrain an alternating-current resistance caused by leakage flux from increasing. In addition, in a reactor or a transformer for a switching power supply, a magnetic core (core) is arranged next to a coil in many cases and hence loss caused by a leakage flux from a magnetic core is likely to be generated.
As a measure of challenge for reducing eddy-current loss generated by a leakage flux, a technology of plating the outer circumference of a copper wire forming a coil with a soft magnetic material such as iron (Fe) or nickel (Ni) is proposed for example. As a result, since a magnetic field generated in another conductive material can pass not through the own conductive material but through a soft magnetic material, the magnetic field acting in the interior of the conductive material can be reduced and an eddy-current loss generated by the magnetic field can be restrained.